The present invention is directed to a fitting suitable for connecting a plastic fluid transmission element to an externally threaded, metallic transmission element.
Fittings for coupling plastic to metal elements are needed in many applications. For example, in commercial sprinkler systems, plastic pipes are used for transmitting water to sprinkler heads, which are typically made of metal. Other applications where plastic to metal transitions occur include hot and cold water pipes into and from hot water heaters, municipal water main connections to residential housing, and swimming pool and spa heaters.
Plastic female threaded fittings are typically used for this purpose. The difficulty with these fittings is that they sometimes fail in use. The failure most often occurs when a metal nipple is threaded into the plastic female threaded fitting and excessive torque is applied. Because of the visco-elastic nature of certain plastic materials, they yield more easily on thread makeup than metal. The circumferential stresses generated can exceed the failure limit of the plastic material. Because of the inherent potential for failure when using plastic female threaded fittings to join metal male threaded fittings, some municipal codes, such as the UPC prohibit the use of female threaded fittings.
Other fittings using O-rings have experienced leakage where inspectors and installers have mistakenly unthreaded the insert out of the coupling.
Accordingly, there is a need for a metal/plastic fitting that is strong, has better resistance to leakage and stress cracking than conventional fittings, and prevents rotation of the insert when torque is applied.